The complement system is part of the innate immune system and includes a set of plasma proteins that act together to attack extracellular forms of pathogens. Activation of the complement system can occur in response to pathogens themselves or to antibodies bound to a pathogen. Upon activation, proteases in the complement system cleave specific proteins to release cytokines and initiate an amplifying cascade of further cleavage reactions. This activation cascade results is massive amplification of the responsee, with cleavage of C3 and C5 by their respective convertases as the terminal activation event. Cleavage of C5 results in formation of two distinct molecules: C5a and C5b. C5b reacts with complement proteins C6, C7, C8 and C9 to effect activation of the cell-killing membrane attack complex (MAC). The membrane attack complex forms transmembrane channels on the surface of pathogen cells, leading to cell lysis and death (Peitsch et al., Curr. Opin. Cell. Biol., 3(4): 710-716 (1991), and C. A. Janeway et al. (eds.), Immunobiology, 5th Ed., Garland Publishing, New York, N.Y. (2001)).
The complement system includes more than 30 proteins and protein fragments, including serum proteins, serosal proteins, and cell membrane receptors. Complement protein 5 (C5) is the fifth component of the complement system, and plays an important role in inflammatory and cell killing processes. The C5 protein is composed of alpha and beta polypeptide chains that are linked by a disulfide bridge. An activation peptide, C5a, which is an anaphylatoxin that possesses potent spasmogenic and chemotactic activity, is derived from the alpha polypeptide via cleavage with a convertase. C5a is short-lived in situ, and in the absence of binding to its cognate receptor, C5a is quickly degraded. Binding of C5a to the C5a receptor (C5aR) initiates a potent local inflammatory and immune response, and this pathway has been targeted for multiple disease indications such as myocardial infarction (MI) and graft-versus-host disease. Strategies for targeting the activity of C5a have included the inhibition of C5 cleavage and soluble C5aR-Fc constructs that compete for C5a binding with the endogenous receptor.
Following cleavage, complement protein C5b initiates formation of the membrane attack complex (MAC) in conjunction with proteins C6 and C7. The assembly process is mediated by the C5 C-terminal interaction domain C345C, which is 800 amino acids distal to the site of proteolytic activation. This site is thought to be critical for binding during MAC initiation because binding of C7 to this domain is essential for nonreversible MAC formation (see, e.g., Tack et al., Biochemistry, 18(8): 1490-1497 (1979); and Muller-Eberhard, H. J., Annual Review of Immunology, 4: 503-528 (1986)).
Inappropriate complement activation and MAC formation has been associated with a number of disease states, including, for example, paroxysmal nocturnal hemoglobinuria (PNH), uveoretinitis, atypical hemolytic uremic syndrome (aHUS), and osteoarthritis (see, e.g., Copland et al., Clinical and Experimental Immunol., 159: 303-314 (2010); Rother et al., Nat. Biotechnol., 25: 1256-1264 (2007); and Wang et al., Nat. Med., 17: 1674-1679 (2011)). Deficiencies in the gene encoding the complement protein C5 lead to complement component C5 deficiency, and also have been linked to increased susceptibility to liver fibrosis and rheumatoid arthritis (see, e.g., Tack et al., supra)
Eculizumab (SOLIRIS™) is a monoclonal antibody directed against the C5 complement protein, and currently is the only pharmaceutical that blocks C5 proteolysis activation, inhibiting formation of both C5a and C5b. Eculizumab is approved for clinical use to treat PNH and aHUS, but is associated with significant immune-suppressive side effects, including an increased risk of meningococcal infections (see, e.g., Dmytrijuk et al., The Oncologist, 13(9): 993-1000 (2008)).
Therefore, there is a need for a C5-binding agent (e.g., an antibody) that binds C5 complement protein with a high affinity, and effectively neutralizes C5b activity and subsequent C5b-dependent MAC formation in vivo, but that does not block C5a activation, thereby avoiding immune-suppressive effects. The invention provides such C5 complement protein-binding agents.